Plasma spray physical vapor deposition deposited in multilayer, multi-microstructure environmental barrier coating

ABSTRACT

An article may include a substrate defining at least one at least partially obstructed surface. The substrate includes at least one of a ceramic or a ceramic matrix composite. The article also may include a multilayer, multi-microstructure environmental barrier coating on the at least partially obstructed substrate. The multilayer, multi-microstructure environmental barrier coating includes a first layer comprising a rare earth disilicate and a substantially dense microstructure; and a second layer on the first layer. The second layer includes a columnar microstructure and at least one of a rare earth monosilicate or a thermal barrier coating composition comprising a base oxide comprising zirconia or hafnia; a primary dopant comprising ytterbia; a first co-dopant comprising samaria; and a second co-dopant comprising at least one of lutetia, scandia, ceria, gadolinia, neodymia, or europia.

This application claims the benefit of U.S. Provisional Application No.62/289,064 filed Jan. 29, 2016, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The disclosure relates to techniques for forming multilayer,multi-microstructure environmental barrier coatings using plasma sprayphysical vapor deposition.

BACKGROUND

Ceramic or ceramic matrix composite (CMC) materials may be useful in avariety of contexts where mechanical and thermal properties areimportant. For example, components of high temperature mechanicalsystems, such as gas turbine engines, may be made from ceramic or CMCmaterials. Ceramic or CMC materials may be resistant to hightemperatures, but some ceramic or CMC materials may react with someelements and compounds present in the operating environment of hightemperature mechanical systems, such as water vapor. Reaction with watervapor may result in the recession of the ceramic or CMC material. Thesereactions may damage the ceramic or CMC material and reduce mechanicalproperties of the ceramic or CMC material, which may reduce the usefullifetime of the component. Thus, in some examples, a ceramic or CMCmaterial may be coated with an environmental barrier coating, which mayreduce exposure of the substrate to elements and compounds present inthe operating environment of high temperature mechanical systems.

SUMMARY

In some examples, the disclosure describes an article that includes asubstrate defining at least one at least partially obstructed surface.The substrate includes at least one of a ceramic or a ceramic matrixcomposite. The article also may include a multilayer,multi-microstructure environmental barrier coating on the at leastpartially obstructed substrate. The multilayer, multi-microstructureenvironmental barrier coating includes a first layer comprising a rareearth disilicate and a substantially dense microstructure and a secondlayer on the first layer. The second layer includes a columnarmicrostructure and at least one of a rare earth monosilicate or athermal barrier coating composition comprising a base oxide comprisingzirconia or hafnia; a primary dopant comprising ytterbia; a firstco-dopant comprising samaria; and a second co-dopant comprising at leastone of lutetia, scandia, ceria, gadolinia, neodymia, or europia.

In some examples, the disclosure describes a system that includes avacuum pump, a vacuum chamber, a plasma spray device, a coating materialsource, and a computing device. The computing device is configured tocontrol the vacuum pump to evacuate the vacuum chamber to high vacuum.The computing device also is configured to control the coating materialsource to provide a first coating material to the plasma spray device ata first feed rate, the first coating material having a compositionselected so that a first layer formed from the first coating materialcomprises a rare earth disilicate, and the first feed rate beingselected to result in a substantially dense microstructure for the firstlayer. Additionally, the computing device is configured to control theplasma spray device to deposit the first layer on a substrate in thevacuum chamber using plasma spray physical vapor deposition, wherein thefirst layer comprises the rare earth disilicate. The computing devicefurther is configured to control the coating material source to providea second coating material to the plasma spray device at a second feedrate, the second coating material having a composition selected so thata second layer formed from the second coating material comprises a rareearth monosilicate or a thermal barrier coating composition comprising abase oxide comprising zirconia or hafnia; a primary dopant comprisingytterbia; a first co-dopant comprising samaria; and a second co-dopantcomprising at least one of lutetia, scandia, ceria, gadolinia, neodymia,or europia. The computing device also is configured to control theplasma spray device to deposit the second layer on the first layer usingplasma spray physical vapor deposition, wherein the second layercomprises the rare earth monosilicate or the thermal barrier coatingcomposition comprising the base oxide comprising zirconia or hafnia; theprimary dopant comprising ytterbia; the first co-dopant comprisingsamaria; and the second co-dopant comprising at least one of lutetia,scandia, ceria, gadolinia, neodymia, or europia.

In some examples, the disclosure describes a method that includescontrolling, by a computing device, a vacuum pump to evacuate the vacuumchamber to high vacuum. The method also includes controlling, by thecomputing device, a coating material source to provide a first coatingmaterial to a plasma spray device at a first feed rate, the firstcoating material having a composition selected so that a first layerfrom the first coating material comprises a rare earth disilicate, andthe first feed rate being selected to result in a substantially densemicrostructure for the first layer. Further, the method includescontrolling, by the computing device, the plasma spray device to depositthe first layer on a substrate in the vacuum chamber using plasma sprayphysical vapor deposition, wherein the first layer comprises the rareearth disilicate. The method additionally includes controlling, by thecomputing device, the coating material source to provide a secondcoating material to the plasma spray device at a second feed rate, thesecond coating material having a composition selected so that a secondlayer formed on the first layer from the second coating materialcomprises a rare earth monosilicate or a thermal barrier coatingcomposition comprising a base oxide comprising zirconia or hafnia; aprimary dopant comprising ytterbia; a first co-dopant comprisingsamaria; and a second co-dopant comprising at least one of lutetia,scandia, ceria, gadolinia, neodymia, or europia. The method alsoincludes controlling, by the computing device, the plasma spray deviceto deposit the second layer on the first layer using plasma sprayphysical vapor deposition, wherein the second layer comprises the rareearth monosilicate or the thermal barrier coating composition comprisingthe base oxide comprising zirconia or hafnia; the primary dopantcomprising ytterbia; the first co-dopant comprising samaria; and thesecond co-dopant comprising at least one of lutetia, scandia, ceria,gadolinia, neodymia, or europia.

In some examples, the disclosure describes a computer readable storagedevice including instructions that, when executed, cause a computingdevice to control a vacuum pump to evacuate the vacuum chamber to highvacuum. The computer readable storage device also includes instructionsthat, when executed, cause the computing device to control a coatingmaterial source to provide a first coating material to a plasma spraydevice at a first feed rate, the first coating material having acomposition selected so that a first layer from the first coatingmaterial comprises a rare earth disilicate, and the first feed ratebeing selected to result in a substantially dense microstructure for thefirst layer. The computer readable storage device further includesinstructions that, when executed, cause the computing device to controlthe plasma spray device to deposit the first layer on a substrate in thevacuum chamber using plasma spray physical vapor deposition, wherein thefirst layer comprises the rare earth disilicate. The computer readablestorage device additionally includes instructions that, when executed,cause the computing device to control the coating material source toprovide a second coating material to the plasma spray device at a secondfeed rate, the second coating material having a composition selected sothat a second layer formed on the first layer from the second coatingmaterial comprises a rare earth monosilicate or a thermal barriercoating composition comprising a base oxide comprising zirconia orhafnia; a primary dopant comprising ytterbia; a first co-dopantcomprising samaria; and a second co-dopant comprising at least one oflutetia, scandia, ceria, gadolinia, neodymia, or europia. The computerreadable storage device also includes instructions that, when executed,cause the computing device to control the plasma spray device to depositthe second layer on the first layer using plasma spray physical vapordeposition, wherein the second layer comprises the rare earthmonosilicate or the thermal barrier coating composition comprising thebase oxide comprising zirconia or hafnia; the primary dopant comprisingytterbia; the first co-dopant comprising samaria; and the secondco-dopant comprising at least one of lutetia, scandia, ceria, gadolinia,neodymia, or europia.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual and schematic diagram illustrating an examplesystem for forming a multilayer, multi-microstructure environmentalbarrier coating using plasma spray physical vapor deposition.

FIG. 2 is a conceptual block diagram illustrating an example articleincluding a substrate and a multilayer, multi-microstructureenvironmental barrier coating including a first, substantially denselayer, and a second, columnar layer.

FIG. 3 is a conceptual block diagram illustrating an example articleincluding a substrate and a multilayer, multi-microstructureenvironmental barrier coating including a first, substantially denselayer, a second, columnar layer, and a third, columnar layer.

FIG. 4 is a conceptual block diagram illustrating an example articleincluding a substrate and a multilayer, multi-microstructureenvironmental barrier coating including a first, substantially denselayer, and a plurality of columnar layers.

FIG. 5 is a flow diagram illustrating an example technique for forming amultilayer, multi-microstructure environmental barrier coating usingplasma spray physical vapor deposition.

FIG. 6 is a scatter diagram illustrating an example relationship betweenexcess silica in a coating material and an amount of silicon in aresulting coating.

FIG. 7 is a cross-sectional picture of an example coating depositedusing PS-PVD as described in this disclosure.

FIG. 8 is a cross-sectional picture of an example coating depositedusing PS-PVD as described in this disclosure.

FIG. 9 is a cross-sectional picture of an example coating depositedusing PS-PVD as described in this disclosure.

DETAILED DESCRIPTION

The disclosure describes systems and techniques for forming amultilayer, multi-microstructure environmental barrier coating (EBC)using plasma spray physical vapor deposition (PS PVD). EBCs protectceramic matrix composite (CMC) substrates based on silicon fromrecession caused by high pressure, high velocity water vapor inenvironments such as combustion environments in gas turbine engines.Some EBCs are prime reliant coatings, meaning that the EBC must remainon the CMC component for the life of the CMC component. Prime reliantEBCs may fulfill multiple, competing design parameters, including highwater vapor stability and thermal expansion compatibility with theunderlying CMC. Additionally, some prime reliant EBCs are used onsurfaces that are in non-line-of-sight relationship with a coatingsource during manufacturing. These surfaces are referred to herein as atleast partially obstructed surfaces. For example, internal surfaces ofgas turbine engine blades, vanes, or bladetracks and areas betweendoublet or triplet vanes in gas turbine engines may not be able to beput into line-of-sight with a coating source during the manufacture ofthe coating.

In some examples, PS PVD may be used to deposit a multilayer,multi-microstructure EBC on surfaces of a CMC, includingnon-line-of-sight (NLOS) or at least partially obstructed surfaces ofthe CMC. Additionally, PS PVD is a flexible process that allowsrelatively easy adjustment of process parameters to result in coatingswith different chemistry, microstructure, or both. In this way, PS PVDmay be used to deposit a multilayer, multi-microstructure EBC thatprovides desired high water vapor stability and thermal expansioncompatibility with the underlying CMC in a relatively fast, relativelycost-effective process.

In some examples, a first layer (closer to the underlying CMC) of themultilayer, multi-microstructure EBC includes at least one rare earthdisilicate and a substantially dense microstructure. A second layer(further from the underlying CMC) of the multilayer,multi-microstructure EBC may include a columnar microstructure, and mayinclude at least one of a rare earth monosilicate or a thermal barriercoating composition comprising rare earth oxide-stabilized hafnia orrare earth oxide-stabilized zirconia. The at least one rare earthdisilicate may have good thermal expansion compatibility with theunderlying CMC, e.g., better than the at least one rare earthmonosilicate or the low thermal conductivity thermal barrier coatingcomposition. Further, by including a substantially dense microstructure,the first layer may provide a diffusion barrier for any water vapor thatreaches the surface of the first layer.

The at least one of the rare earth monosilicate or the thermal barriercoating composition comprising rare earth oxide-stabilized hafnia orrare earth oxide-stabilized zirconia may have good water vaporstability, e.g., better than the rare earth disilicate. The columnarmicrostructure of the second layer may mitigate stress due to thermalexpansion mismatch between the second layer and the first layer.Further, presence of the second layer may result in reduced velocity ofwater vapor at the surface of the first layer, reducing or substantiallypreventing recession of the first layer.

FIG. 1 is a conceptual and schematic diagram illustrating an examplesystem 10 for forming a multilayer, multi-microstructure EBC 18 using PSPVD. System 10 includes a vacuum chamber 12, which encloses a stage 14and a plasma spray device 20. System 10 also includes a vacuum pump 24,a coating material source 26, and a computing device 22. A substrate 16is disposed in vacuum chamber 12 and includes multilayer,multi-microstructure EBC 18.

Vacuum chamber 12 may substantially enclose (e.g., enclose or nearlyenclose) stage 14, substrate 16, and plasma spray device 20. Vacuumchamber 12 is fluidically connected to at least one vacuum pump 24,which is operable to pump fluid (e.g., gases) from the interior ofvacuum chamber 12 to establish a vacuum in vacuum chamber 12. In someexamples, vacuum pump 24 may include multiple pumps or multiple stagesof a pump, which together may evacuate vacuum chamber 12 to high vacuum.For example, vacuum pump 24 may include at least one of a scroll pump, ascrew pump, a roots pump, a turbomolecular pump, or the like. As usedherein, high vacuum may refer to pressures of less than about 10 torr(less than about 1.33 kilopascals (kPa)). In some examples, the pressurewithin vacuum chamber 12 during the PS-PVD technique may be betweenabout 0.5 torr (about 66.7 pascals) and about 10 torr (about 1.33 kPa).

In some examples, during the evacuation process, vacuum chamber 12 maybe backfilled with a substantially inert atmosphere (e.g., helium,argon, or the like), then the substantially inert gases removed duringsubsequent evacuation to the target pressure (e.g., high vacuum). Inthis way, the gas molecules remaining in vacuum chamber 12 under highvacuum may be substantially inert, e.g., to substrate 16 and multilayer,multi-microstructure EBC 18.

In some examples, stage 14 may be configured to selectively position andrestrain substrate 16 in place relative to stage 14 during formation ofmultilayer, multi-microstructure EBC 18. In some examples, stage 14 ismovable relative to plasma spray device 20. For example, stage 14 may betranslatable and/or rotatable along at least one axis to positionsubstrate 16 relative to plasma spray device 20. Similarly, in someexamples, plasma spray device 20 may be movable relative to stage 14 toposition plasma spray device 20 relative to substrate 16.

Plasma spray device 20 includes a device used to generate a plasma 28for use in the PS PVD technique. For example, plasma spray device 20 mayinclude a plasma spray gun including a cathode and an anode separated bya plasma gas channel. As the plasma gas flows through the plasma gaschannel, a voltage may be applied between the cathode and anode to causethe plasma gas to form the plasma 28. In some examples, the coatingmaterial may be injected inside plasma spray device 20 such that thecoating material flows through part of the plasma gas channel. In someexamples, the coating material may be introduced to the plasma externalto plasma spray device 20, as shown in FIG. 1. In some examples, thecoating material may be a relatively fine powder (e.g., an averageparticle size of less than about 5 micrometers) to facilitatevaporization of the coating material by the plasma. In some examples,the relatively fine powder may be agglomerated into a composite powderthat serves as the material fed to plasma spray device 20. The compositepowder may have a particle size that is larger than the relatively finepowder.

Coating material source 26 may include at least one source of materialwhich is injected into the plasma 28 generated by plasma spray device 20and deposited in a layer of multilayer, multi-microstructure EBC 18 onsubstrate 16. In some examples, the material may be in powder form, andmay be supplied by coating material source 26 carried by a fluid, suchas air, an inert gas, or the like. In some examples, system 10 mayinclude multiple coating material sources, e.g., one coating materialsource for each layer of multilayer, multi-microstructure EBC 18.

The multilayer, multi-microstructure EBC 18 may include a first layerincluding at least one rare earth disilicate (RE₂Si₂O₇, where RE is arare earth element selected from the group consisting of lutetium,ytterbium, thulium, erbium, holmium, dysprosium, terbium, gadolinium,europium, samarium, promethium, neodymium, praseodymium, cerium,lanthanum, yttrium, and scandium). The multilayer, multi-microstructureEBC 18 also may include a second layer including at least one rare earthmonosilicate (RE₂SiO₅, where RE is a rare earth element selected fromthe group consisting of lutetium, ytterbium, thulium, erbium, holmium,dysprosium, terbium, gadolinium, europium, samarium, promethium,neodymium, praseodymium, cerium, lanthanum, yttrium, and scandium) or athermal barrier coating composition comprising rare earthoxide-stabilized hafnia or rare earth oxide-stabilized zirconia. Inexamples in which the first layer includes at least one rare earthdisilicate and the second layer includes at least one rare earthmonosilicate, the rare earth element in the rare earth silicates may bethe same or may be different. As such, in some examples, system 10 mayinclude a single coating material source 26. In other examples, system10 may include multiple coating material sources 26, e.g., one coatingmaterial source for each distinct layer chemistry.

In some examples, the coating material for a layer including at leastone rare earth disilicate or at least one rare earth monosilicate mayinclude particles, such as particles including a rare earth oxide,particles that include silica, or both, where the particles includingrare earth oxide are separate from the particles including silica, andare mechanically mixed in a powder mixture. In other examples, thecoating material for a layer including at least one rare earthdisilicate or at least one rare earth monosilicate may include particlesin which rare earth oxide and silica are chemically reacted in the formof rare earth monosilicate or rare earth disilicate. In other examples,the particles including rare earth oxide may be agglomerated withparticles including silica to form larger particles. For example,particles of rare earth oxide and particles of silica may be mixed andagglomerated such that the agglomerated particles include a ratio ofmoles of rare earth oxide to moles of silica in an approximatelystoichiometric amount for the selected type of silicate (e.g., a rareearth monosilicate or a rare earth disilicate).

In some examples, a coating material provided by coating material source26 may include additional and optional constituents of a layer ofmultilayer, multi-microstructure EBC 18. For example, the additional andoptional constituents in a layer including at least one rare earthdisilicate or at least one rare earth monosilicate may include alumina,an alkali metal oxide, an alkaline earth metal oxide, TiO₂, Ta₂O₅,HfSiO₄, or the like. The additive may be added to the layer to modifyone or more desired properties of the layer. For example, the additivecomponents may increase or decrease the reaction rate of the layer withcalcia-magnesia-alumina-silicate (CMAS; a contaminant that may bepresent in intake gases of gas turbine engines), may modify theviscosity of the reaction product from the reaction of CMAS andconstituent(s) of the layer, may increase adhesion of the layer to anadjacent layer, may increase the chemical stability of the layer, maydecrease steam oxidation rate, or the like.

Computing device 22 may include, for example, a desktop computer, alaptop computer, a workstation, a server, a mainframe, a cloud computingsystem, or the like. Computing device 22 may include or may be one ormore processors, such as one or more digital signal processors (DSPs),general purpose microprocessors, application specific integratedcircuits (ASICs), field programmable logic arrays (FPGAs), or otherequivalent integrated or discrete logic circuitry. Accordingly, the term“processor,” as used herein may refer to any of the foregoing structureor any other structure suitable for implementation of the techniquesdescribed herein. In addition, in some examples, the functionality ofcomputing device 22 may be provided within dedicated hardware and/orsoftware modules.

Computing device 22 is configured to control operation of system 10,including, for example, stage 14, plasma spray device 20, and/or vacuumpump 24. Computing device 22 may be communicatively coupled to at leastone of stage 14, plasma spray device 20, and/or vacuum pump 24 usingrespective communication connections. Such connections may be wirelessand/or wired connections.

Computing device 22 may be configured to control operation of stage 14and/or plasma spray device 20 to position substrate 16 relative toplasma spray device 20. For example, as described above, computingdevice 22 may control plasma spray device 20 to translate and/or rotatealong at least one axis to position substrate 16 relative to plasmaspray device 20.

As described above, system 10 may be configured to perform a PS PVDtechnique to deposit multilayer, multi-microstructure EBC 18 onsubstrate 16. In some examples, substrate 16 may include component of ahigh temperature mechanical system, such as a gas turbine engine. Forexample, substrate 16 may be part of a seal segment, a blade track, anairfoil, a blade, a vane, a combustion chamber liner, or the like. Insome examples, substrate may include a ceramic or a CMC. Example ceramicmaterials may include, for example, silicon carbide (SiC), siliconnitride (Si₃N₄), alumina (Al₂O₃), aluminosilicate, silica (SiO₂),transition metal carbides and silicides (e.g. WC, Mo₂C, TiC, MoSi₂,NbSi₂, TiSi₂), or the like. In some examples, substrate 16 additionallymay include silicon metal, carbon, or the like. In some examples,substrate 16 may include mixtures of two or more of SiC, Si₃N₄, Al₂O₃,aluminosilicate, silica, silicon metal, carbon, or the like.

In examples in which substrate 16 includes a CMC, substrate 16 includesa matrix material and a reinforcement material. The matrix materialincludes a ceramic material, such as, for example, silicon metal, SiC,or other ceramics described herein. The CMC further includes acontinuous or discontinuous reinforcement material. For example, thereinforcement material may include discontinuous whiskers, platelets,fibers, or particulates. As other examples, the reinforcement materialmay include a continuous monofilament or multifilament weave. In someexamples, the reinforcement material may include SiC, C, other ceramicmaterials described herein, or the like. In some examples, substrate 16includes a SiC—SiC ceramic matrix composite.

System 10 may be used to perform PS PVD to deposit multilayer,multi-microstructure EBC 18 on surfaces of substrate 16, including NLOSsurfaces of substrate 16. PS PVD is a flexible process that allowsrelatively easy adjustment of process parameters to result in coatingswith different chemistry, microstructure, or both. In this way, system10 may utilize PS PVD to deposit multilayer, multi-microstructure EBC 18that provides desired high water vapor stability and thermal expansioncompatibility with the underlying CMC in a relatively fast, relativelycost-effective process.

Computing device 22 may be configured to control operation of system 10(e.g., vacuum pump 24, plasma spray device 20, and coating materialsource 26) to perform PS PVD to deposit the EBC 18. PS PVD may operateat low operating pressures, such as between about 0.5 torr and about 10torr. In some examples, the temperatures of the plasma may be greaterthan about 6000 K, which may vaporize the coating material. Because thevaporized coating material is carried by a gas stream, PS PVD may allowdeposition multilayer, multi-microstructure EBC 18 on surfaces ofsubstrate 16 that are not in line-of-sight relationship with plasmaspray device 20, unlike thermal spray processes, such as air plasmaspraying. Further, a deposition rate (e.g., thickness of coatingdeposited per unit time) may be greater for PS PVD than for other vaporphase deposition processes, such as chemical vapor deposition orphysical vapor deposition, which may result in PS PVD being a moreeconomical coating technique.

In some examples, multilayer, multi-microstructure EBC 18 includes afirst layer (closer to the underlying CMC) that includes at least onerare earth disilicate and a substantially dense microstructure. The atleast one rare earth disilicate may have good thermal expansioncompatibility with the underlying CMC, e.g., better than the at leastone rare earth monosilicate or the low thermal conductivity thermalbarrier coating composition. Further, by including a substantially densemicrostructure, the first layer may substantially prevent (e.g., preventor nearly prevent) environmental species such as water vapor, oxygen,molten salt, or calcia-magnesia-alumina-silicate (CMAS) deposits fromcontacting substrate 16 and degrading the material structure ofsubstrate 16. For example, water vapor may react with a substrate 16including a CMC and volatilize silica or alumina components in substrate16. Consequently, a first layer which is substantially dense (e.g.,substantially nonporous) may provide protection to substrate 16 bypreventing water vapor from contacting and reacting with substrate 16.In some examples, a layer with a substantially dense microstructure mayhave a porosity of less than about 10 vol. %, such as, e.g., less thanabout 5 vol. %, where porosity is measured as a percentage of porevolume divided by total layer volume.

System 10 may utilize PS PVD to deposit the first layer. For example,the rate at which coating material is fed by coating material source 26into plasma 28 may affect the amount of the coating material that isvaporized by plasma 28. A higher rate of coating material being fed intoplasma 28 may reduce the amount of the coating material that isvaporized by plasma 28. When substantially all of the coating materialis vaporized, the resulting deposited layer may be substantially dense,while when less coating material is vaporized, the resulting depositedlayer may have a columnar microstructure. Hence, by reducing the rate ofcoating material fed by coating material source 26 into plasma 28,computing device 22 may cause a first layer having a substantially densemicrostructure to be deposited on substrate 16.

In some examples, multilayer, multi-microstructure EBC 18 also includesa second layer (further from the underlying CMC) that may include acolumnar microstructure, and may include at least one of a rare earthmonosilicate or a thermal barrier coating composition comprising rareearth oxide-stabilized hafnia or rare earth oxide-stabilized zirconia.The at least one of the rare earth monosilicate or the thermal barriercoating composition comprising rare earth oxide-stabilized hafnia orrare earth oxide-stabilized zirconia may have good water vaporstability, e.g., better than the at least one rare earth disilicate. Acolumnar microstructure may have microcracks or microgaps that extendthrough at least a portion of the layer in a direction that issubstantially orthogonal to the plane defined by the layer surface.Because of the microgaps, a columnar microstructure may have enhancedmechanical compliance under thermal cycling or when a temperaturegradient exists, such as when a high-temperature system is firstengaged. Additionally, a layer having a columnar microstructure mayprovide improved thermal protection to substrate 16 compared to a layerthat is substantially nonporous. While not wishing to be bound bytheory, the microcracks or microgaps may provide scattering sites forthermal energy-carrying phonons, which may lower an effective thermalconductivity of a layer having a columnar microstructure compared to asubstantially nonporous layer of a similar composition. Further,presence of the second layer may result in reduced velocity of watervapor at the surface of the first layer, reducing or substantiallypreventing recession of the first layer.

In some examples, thermal protection and mechanical compliance are notthe only benefits of a columnar microstructure EBC. An EBC having acolumnar microstructure may also exhibit enhanced erosion resistance andenhanced sintering resistance relative to an EBC that does not include acolumnar microstructure.

System 10 may utilize PS PVD to deposit the second layer. For example,computing device 22 may cause coating material source 26 to feed coatingmaterial into plasma 28 at higher rate so that less than all of thecoating material is vaporized by plasma 28. When less coating materialis vaporized, the resulting deposited layer may have a columnarmicrostructure. Hence, by increasing the rate of coating material fed bycoating material source 26 into plasma 28, computing device 22 may causea second layer having a columnar microstructure to be deposited onsubstrate 16.

In this way, system 10 may utilize PS PVD to deposit multilayer,multi-microstructure EBC 18 on substrate 16, where multilayer,multi-microstructure EBC 18 includes a first layer including at leastone rare earth disilicate and a substantially dense microstructure and asecond layer including a columnar microstructure and at least one rareearth monosilicate or a thermal barrier coating composition comprisingrare earth oxide-stabilized hafnia or rare earth oxide-stabilizedzirconia. In some examples, system 10 may utilize PS PVD to depositmultilayer, multi-microstructure EBC 18 on NLOS surfaces of substrate16, which may not be possible with EB-PVD (electron beam physical vapordeposition) techniques often used to deposit columnar coating layers.

In some examples in which a layer of multilayer, multi-microstructureEBC 18 includes at least one rare earth disilicate or at least one rareearth monosilicate, the coating material may include excess silicacompared to the desired amount of silica in the layer of multilayer,multi-microstructure EBC 18 (e.g., in the monosilicate or disilicate).In some examples, the excess silica may be mixed in the coating materialas a separate powder. In other examples, the excess silica may be partof an agglomerate with the rare earth oxide.

The excess silica in the coating source may facilitate formation of alayer with a desired composition. As described above, silica may have ahigher vapor pressure than rare earth oxides, at a given pressure andtemperature. This may result in silica being more likely to be lost viavolatilization during the processing, such that silica deposits in thelayer in a lower ratio than the ratio of silica in the coating material.Thus, by including excess silica in a predetermined amount, a layer maybe formed with a desired amount of silica. For example, the excessamount of silica may be selected such that the ratio of silica to earthoxide deposited in the first layer is substantially the same as astoichiometric ratio of the desired rare earth disilicate. In otherexamples, the amount of silica in the coating material may be selectedto result in a predetermined amount of excess silica or excess rareearth oxide in the layer being deposited compared to a stoichiometricratio of rare earth oxide to silica in the desired rare earth silicate.This may result in a selected amount of free silica or free rare earthoxide in the layer of multilayer, multi-microstructure EBC 18.

The amount of excess silica included in the coating material may dependon the desired composition of the layer in multilayer,multi-microstructure EBC 18, and may be based on experimental testing.For example, a first coating material having a first ratio of silica torare earth oxide may be formed and a coating deposited from the coatingmaterial using PS PVD. The composition of the resulting coating may bedetermined, and the ratio of silica to rare earth oxide (e.g., in theform of a rare earth silicate) in the coating may be compared to theratio of silica to rare earth oxide in the coating material. Thisprocess may be repeated to determine an amount of excess silica toinclude in the coating material to form the layer in multilayer,multi-microstructure EBC 18 with a desired composition.

Multilayer, multi-microstructure EBC 18 may include a first layer and asecond layer, and also may include one or more optional layers, as shownin FIGS. 2-4. FIG. 2 is a conceptual block diagram illustrating anexample article 30 including a substrate 32 and a coating 34 thatincludes a multilayer, multi-microstructure EBC including a first layer38 and a second layer 40. Substrate 32 may include any of the materialsdescribed above with respect to substrate 16 of FIG. 1.

Coating 34 also includes an optional bond coat 36. Bond coat 36 mayinclude, for example, silicon metal, alone, or mixed with at least oneother constituent. For example, bond coat 36 may include silicon metaland at least one of a transition metal carbide, a transition metalboride, a transition metal nitride, mullite (aluminum silicate,Al₆Si₂O₁₃), silica, a silicide, an oxide (e.g., silicon oxide, a rareearth oxide, an alkali oxide, an alkali earth metal oxide, or the like),a silicate (e.g., a rare earth silicate or the like), or the like. Insome examples, the additional constituent(s) may be substantiallyhomogeneously mixed with silicon metal. In other examples, theadditional constituent(s) may form a second phase distinct from thesilicon metal phase.

As described above, first layer 38 includes at least one rare earthdisilicate and a substantially dense microstructure. A rare earthdisilicate is a compound formed by chemically reacting a rare earthoxide and silica in a particular stoichiometric ratio (1 mole rare earthoxide and 2 moles silica) under sufficient conditions (e.g., heat and/orpressure) to cause the rare earth oxide and the silica to react. A rareearth disilicate is chemically distinct from a mixture of free rareearth oxide and free silica. For example, a rare earth disilicate hasdifferent chemical and physical properties than a mixture of free rareearth oxide and free silica.

In some examples, first layer 38 may consist essentially of or consistof the at least one rare earth disilicate. In other examples, firstlayer 38 may include the at least one rare earth disilicate and at leastone other element or compound. For example, first layer 38 may includethe rare earth disilicate and at least one of free rare earth oxide,free silica, or rare earth monosilicate.

Additionally and optionally, first layer 38 may include at least onedopant. The at least one dopant may include at least one of alumina(Al₂O₃), at least one alkali oxide, or at least one alkaline earthoxide. In some examples, first layer 38 may include between about 0.1wt. % and about 5 wt. % of the at least one dopant. In some examples inwhich the at least one dopant includes alumina, first layer 38 mayinclude between about 0.5 wt. % and about 3 wt. % alumina or betweenabout 0.5 wt. % and about 1 wt. % alumina. In some examples in which theat least one dopant includes the at least one alkali oxide, first layer38 may include between about 0.1 wt. % and about 1 wt. % of the at leastone alkali oxide. In some examples in which the at least one dopantincludes the at least one alkaline earth oxide, first layer 38 mayinclude between about 0.1 wt. % and about 1 wt. % of the at least onealkaline earth oxide. The at least one dopant may affect chemical andphysical properties of first layer 38, including, for example, steamoxidation resistance, calcia-magnesia-alumina-silicate (CMAS)resistance, thermal expansion coefficient, and the like.

First layer 38 includes a substantially dense microstructure. In someexamples, first layer 38 with a substantially dense microstructure mayhave a porosity of less than about 10 vol. %, such as, e.g., less thanabout 5 vol. %, where porosity is measured as a percentage of porevolume divided by total layer volume.

Second layer 40 includes a columnar microstructure and at least one rareearth monosilicate or a thermal barrier coating composition including arare earth oxide-stabilized zirconia or a rare earth oxide-stabilizedhafnia. A columnar microstructure may have microcracks or microgaps thatextend through at least a portion of the layer in a direction that issubstantially orthogonal to the plane defined by the layer surface.Because of the microgaps, a columnar microstructure may have enhancedmechanical compliance under thermal cycling or when a temperaturegradient exists, such as when a high-temperature system is firstengaged. Additionally, a layer having a columnar microstructure mayprovide improved thermal protection to substrate 16 compared to a layerthat is substantially nonporous.

A rare earth monosilicate is a compound formed by chemically reacting arare earth oxide and silica in a particular stoichiometric ratio (1 molerare earth oxide and 1 mole silica) under sufficient conditions (e.g.,heat and/or pressure) to cause the rare earth oxide and the silica toreact. A rare earth monosilicate is chemically distinct from a mixtureof free rare earth oxide and free silica. For example, a rare earthmonosilicate has different chemical and physical properties than amixture of free rare earth oxide and free silica.

In some examples, second layer 40 may consist essentially of or consistof the at least one rare earth monosilicate. In other examples, secondlayer 40 may include the at least one rare earth monosilicate and atleast one other element or compound. For example, second layer 40 mayinclude the rare earth monosilicate and at least one of free rare earthoxide, free silica, or rare earth disilicate.

The thermal barrier coating composition including a rare earthoxide-stabilized zirconia or a rare earth oxide-stabilized may include abase oxide, a primary dopant, a first co-dopant, and a second co-dopant.In some examples, the primary dopant, first co-dopant, and secondco-dopant may be rare earth oxides. The base oxide may be zirconia orhafnia. The primary dopant may be ytterbia, the first co-dopant samaria,and the second co-dopant may be at least one of lutetia, scandia, ceria,gadolinia, neodymia, or europia. Additionally, in some examples, thethermal barrier coating composition may be essentially free of yttria.

The inclusion of rare earth oxides such as ytterbia, samaria, lutetia,scandia, ceria, gadolinia, neodymia, europia, and the like as dopantsmay help decrease the thermal conductivity (by conduction) of thethermal barrier coating composition. While not wishing to be bound byany specific theory, the inclusion of at least one of these dopantoxides in the thermal barrier coating composition may reduce thermalconductivity through one or more mechanisms, as follows.

A first proposed mechanism of reducing thermal conductivity includesintroducing lattice imperfections into the crystal structure of thethermal barrier coating composition. Lattice imperfections includedefects in the crystalline lattice of the thermal barrier coatingcomposition. The defects may be caused by the incorporation of dopantswith differing ionic radii, different atomic weight, or differentcrystalline lattice types. Phonon scattering decreases the thermalconductivity of second layer 40 by reducing the mean free path of aphonon (i.e., the average distance the phonon travels between scatteringsites).

Heavier rare earth oxide dopants are expected to lower the thermalconductivity more than lighter rare earth oxide dopants. For example,rare earth oxides including ytterbia, lutetia, gadolinia, samaria,neodymia, europia, and the like are expected to more effectively lowerthe thermal conductivity of the thermal barrier coating composition thanyttria.

Inclusion of certain rare earth elements in the thermal barrier coatingcomposition may also decrease the extent to which second layer 40sinters at a given temperature. For example, incorporating rare earthelements with a larger ionic radius than yttrium can decrease the amountof sintering at a given temperature. While not wishing to be bound byany theory, a larger ionic radius can lead to a lower diffusioncoefficient at a given temperature. As sintering is primarily adiffusion-related process, a lower diffusion coefficient lowers theamount of sintering at a given temperature.

Minimizing or eliminating sintering may significantly improve thestability of the thermal conductivity of second layer 40 over theservice life of second layer 40. Thermal conductivity of second layer 40is lowered by forming second layer 40 as a columnar structure. Thecolumnar structure of second layer 40 reduces the thermal conductivityby reducing the area through which heat is conducted and by providing alarge refractive index difference between the gaps and the material fromwhich second layer 40 is made, which can reduce heat transfer byradiation. Sintering reduces the gaps in the structure, and thusincreases the thermal conductivity (via both radiation and conduction)of second layer 40.

In order to accomplish at least some of the desired properties mentionedabove, second layer 40, when including the thermal barrier coatingcomposition, may include a composition that includes a base oxide, aprimary dopant, a first co-dopant, and a second co-dopant. In someexamples, second layer 40 may consist essentially of the thermal barriercoating composition, which may consist essentially of the base oxide,the primary dopant, the first co-dopant, and the second co-dopant. Inthe current disclosure, to “consist essentially of” means to consist ofthe listed element(s) or compound(s), while allowing the inclusion ofimpurities present in small amounts such that the impurities do notsubstantially affect the properties of the listed element or compound.For example, the purification of many rare earth elements is difficult,and thus the nominal rare earth element may include small amounts ofother rare earth elements. This mixture is intended to be covered by thelanguage “consist essentially of.”

The base oxide may include or consist essentially of zirconia and/orhafnia. The primary dopant may include or consist essentially ofytterbia. The first co-dopant may include or consist essentially ofsamaria, and second co-dopant may include or consist essentially of atleast one of lutetia, scandia, ceria, gadolinia, neodymia, or europia.In some examples, second layer 40 may include zirconia and/or hafnia incombination with additive elements or compounds such that at least someof the stabilized zirconia or hafnia forms a metastable tetragonal-primecrystalline phase, a cubic crystalline phase, or a compound phase(RE₂Zr₂O₇ or RE₂Hf₂O₇, where RE is a rare earth element).

In some examples, second layer 40 may consist essentially of a baseoxide, a primary dopant, a first co-dopant, and a second co-dopant. Thebase oxide may be selected from zirconia, hafnia, and combinationsthereof. The primary dopant may consist essentially of ytterbia. Thefirst co-dopant may consist essentially of samaria. The second co-dopantmay be selected from at least one of lutetia, scandia, ceria, gadolinia,neodymia, or europia. Second layer 40 may be essentially free of yttria.

In some examples, the primary dopant is present in second layer 40 in anamount greater than either the first co-dopant or the second co-dopant.In various examples, the primary dopant may be present in an amount lessthan, equal to, or greater than the total amount of the first co-dopantand the second co-dopant.

In some examples, second layer 40 includes between about 2 mol. % andabout 40 mol. % of the primary dopant. In other examples, second layer40 includes between about 20 mol. % and about 40 mol. % of the primarydopant, between about 2 mol. % and about 20 mol. % of the primarydopant, between about 2 mol. % and about 10 mol. % of the primarydopant, between about 2 mol. % and about 5 mol. % of the primary dopant.

In some examples, second layer 40 includes between about 0.1 mol. % andabout 20 mol. % of the first co-dopant. In other examples, second layer40 includes between about 10 mol. % and about 20 mol. % of the firstco-dopant, between about 2 mol. % and about 10 mol. % of the firstco-dopant or between about 0.5 mol. % and about 3 mol. % of the firstco-dopant.

In some examples, second layer 40 includes between about 0.1 mol. % andabout 20 mol. % of the second co-dopant. In other examples, second layer40 includes between about 10 mol. % and about 20 mol. % of the secondco-dopant, between about 2 mol. % and about 10 mol. % of the secondco-dopant, or between about 0.5 mol. % and about 3 mol. % of the secondco-dopant.

The composition of second layer 40 provides a desired phaseconstitution. For a second layer 40 that includes zirconia and/orhafnia, a primary dopant, a first co-dopant, and a second co-dopant,accessible phase constitutions include metastable tetragonal-prime,cubic, and compound (RE₂Zr₂O₇ and RE₂Hf₂O₇, where RE is a rare earthelement). To achieve a RE₂O₃—ZrO₂ (and/or HfO₂) compound phaseconstitution, second layer 40 includes between about 20 mol. % and about40 mol. % primary dopant, between about 10 mol. % and about 20 mol. %first co-dopant, between about 10 mol. % and about 20 mol. % secondco-dopant, and the balance base oxide (hafnia and/or zirconia) and anyimpurities present. To achieve a cubic phase constitution, second layer40 includes between about 4 mol. % and about 10 mol. % primary dopant,between about 1 mol. % and about 5 mol. % first co-dopant, between about1 mol. % and about 5 mol. % second co-dopant, and a balance base oxide(zirconia and/or hafnia) and any impurities present. In some examples,to achieve a metastable tetragonal phase constitution, second layer 40includes between about 2 mol. % and about 5 mol. % primary dopant,between about 0.5 mol. % and about 3 mol. % first co-dopant, betweenabout 0.5 mol. % and about 3 mol. % second co-dopant, and a balance baseoxide and any impurities present.

Additionally and optionally, second layer 40 may include at least onedopant. The at least one dopant may include at least one of alumina(Al₂O₃), at least one alkali oxide, or at least one alkaline earthoxide. In some examples, second layer 40 may include between about 0.1wt. % and about 5 wt. % of the at least one dopant. In some examples inwhich the at least one dopant includes alumina, second layer 40 mayinclude between about 0.5 wt. % and about 3 wt. % alumina or betweenabout 0.5 wt. % and about 1 wt. % alumina. In some examples in which theat least one dopant includes the at least one alkali oxide, second layer40 may include between about 0.1 wt. % and about 1 wt. % of the at leastone alkali oxide. In some examples in which the at least one dopantincludes the at least one alkaline earth oxide, second layer 40 mayinclude between about 0.1 wt. % and about 1 wt. % of the at least onealkaline earth oxide. The at least one dopant may affect chemical andphysical properties of second layer 40, including, for example, steamoxidation resistance, calcia-magnesia-alumina-silicate (CMAS)resistance, thermal expansion coefficient, and the like.

In some examples, multilayer, multi-microstructure EBC may include morethan two layers. For example, FIG. 3 is a conceptual block diagramillustrating an example article 50 that includes a substrate 32 and amultilayer, multi-microstructure environmental barrier coating 52including a first, substantially dense layer 54, a second, columnarlayer 56, and a third, columnar layer 58. Substrate 32 is the same assubstrate 32 of FIG. 2.

First, substantially dense layer 54 may be similar to or substantiallythe same as first layer 38 of FIG. 2. For example, first, substantiallydense layer 54 may include a rare earth disilicate. Second, columnarlayer 56 may include at least one rare earth monosilicate and a columnarmicrostructure, as described with respect to second layer 40 of FIG. 2.Third, columnar layer 58 may include a thermal barrier coatingcomposition including at least one of rare earth oxide-stabilizedzirconia or rare earth oxide-stabilized hafnia and a columnarmicrostructure, as described with respect to second layer 40 of FIG. 2.

The rare earth monosilicate in second, columnar layer 56 may have acoefficient of thermal expansion that is greater than the coefficient ofthermal expansion of the rare earth disilicate in first, substantiallydense layer 54. Moreover, the rare earth monosilicate in second,columnar layer 56 may have a coefficient of thermal expansion that isless than the coefficient of thermal expansion of the thermal barriercoating composition including at least one of rare earthoxide-stabilized zirconia or rare earth oxide-stabilized hafnia inthird, columnar layer 58. In this way, second, columnar layer 56 may bea layer with an intermediate coefficient of thermal expansion, which mayresult in reduced stresses within multilayer, multi-microstructureenvironmental barrier coating 52 during thermal cycling than if third,columnar layer 58 were directly adjacent to first, substantially denselayer 54. The thermal barrier coating composition including at least oneof rare earth oxide-stabilized zirconia or rare earth oxide-stabilizedhafnia in third, columnar layer 58 may have a lower thermal conductivityand better water vapor stability at very high temperatures than the rareearth monosilicate in second, columnar layer 56.

FIG. 4 is a conceptual block diagram illustrating an example article 60including a substrate 32 and a multilayer, multi-microstructureenvironmental barrier coating 62 including a first, substantially denselayer 64, and a plurality of columnar layers 66 a, 66 b, 68 a, and 68 b.Substrate 32 is the same as substrate 32 of FIG. 2.

First, substantially dense layer 64 may be similar to or substantiallythe same as first layer 38 of FIG. 2. For example, first, substantiallydense layer 64 may include a rare earth disilicate. First set ofcolumnar layers 66 a and 66 b may include at least one rare earthmonosilicate and a columnar microstructure, as described with respect tosecond layer 40 of FIG. 2. Second set of columnar layers 68 a and 68 bmay include a thermal barrier coating composition including at least oneof rare earth oxide-stabilized zirconia or rare earth oxide-stabilizedhafnia and a columnar microstructure, as described with respect tosecond layer 40 of FIG. 2.

By alternating layers of first set of columnar layers 66 and layers ofsecond set of columnar layers 68, interfaces will be formed betweenadjacent layers. These interfaces provide phonon scattering, whichreduces thermal conductivity of multilayer, multi-microstructureenvironmental barrier coating 62.

FIG. 5 is a flow diagram illustrating an example technique for forming acoating that includes a multilayer, multi-microstructure environmentalbarrier coating using PS PVD. The technique of FIG. 5 will be describedwith respect to system 10 of FIG. 1 and article 30 of FIG. 2 for ease ofdescription only. A person having ordinary skill in the art willrecognize and appreciate that the technique of FIG. 5 may be implementedusing systems other than system 10 of FIG. 1, may be used to formarticles other than article 30 of FIG. 2 (such as article 50 of FIG. 3or article 60 of FIG. 4), or both.

The technique of FIG. 5 may include, controlling, by computing device22, vacuum pump 24 to evacuate vacuum chamber 12 to a high vacuum (72).As described above, vacuum pump 24 may be used to evacuate vacuumchamber 12 to high vacuum, e.g., less than about 10 torr (about 1.33kPa), or between about 0.5 torr (about 66.7 pascals) and about 10 torr(about 1.33 kPa). In some examples, computing device 22 may controlvacuum pump 24 and a source of substantially inert gas (e.g., helium,argon, or the like) to evacuate vacuum chamber 12 in multiplepump-downs. For example, computing device 22 may control vacuum pump 24to evacuate vacuum chamber 12 of the atmosphere present when substrate16 is placed in vacuum chamber 12. Computing device 22 then may controlthe source of the substantially inert gas to fill vacuum chamber 12 withthe substantially inert gas. Computing device 22 may control vacuum pump24 to evacuate vacuum chamber 12 of the substantially inert gas (andremaining atmosphere). In some examples, computing device 22 may controlthe source of the substantially inert gas and vacuum pump 24 to fill andevacuate vacuum chamber 12 at least one time (e.g., a plurality oftimes) to substantially remove reactive gases from vacuum chamber 12 andleave substantially only inert gases such as helium, argon, or the likein vacuum chamber 12.

The technique of FIG. 5 also may include, controlling, by computingdevice 22, coating material source 26 to provide a first coatingmaterial to plasma spray device 20 at a first feed rate (74). Asdescribed above, the first coating material may include silica and atleast one rare earth oxide. The amount of the at least one rare earthoxide and the amount of silica may be selected so that first layer 38deposited from the first coating material includes a predetermined ratioof the at least one rare earth oxide and silica. In some examples, dueto the differences in vapor pressure between rare earth oxides andsilica, the ratio of the at least one rare earth oxide and silica in thefirst coating material provided by coating material source 26 mayinclude additional silica compared to the composition of first layer 38,as described above.

The excess silica in the coating source may facilitate formation offirst layer 38 including a rare earth disilicate. As described above,silica may have a higher vapor pressure than some rare earth oxides, ata given pressure and temperature. This may result in silica being morelikely to be lost via volatilization during the processing, such thatsilica deposits in first layer 38 in a lower ratio than the ratio ofsilica to rare earth oxide in the first coating material. Thus, byincluding excess silica in a predetermined amount, first layer 38 may beformed with a desired amount of silica. For example, the excess amountof silica may be selected such that the ratio of silica to rare earthoxide deposited in first layer 38 is substantially the same as astoichiometric ratio of the rare earth disilicate. In other examples,the amount of silica in the coating material may be selected to resultin a predetermined amount of excess silica or excess rare earth oxide infirst layer 38 compared to a stoichiometric ratio of rare earth oxide tosilica in the rare earth disilicate. This may result in a selectedamount of free silica or free rare earth oxide in first layer 38.

The first feed rate may be selected so PS PVD of the first coatingmaterial results in first layer 38 including a substantially densemicrostructure. In some examples, a layer with a substantially densemicrostructure may have a porosity of less than about 10 vol. %, suchas, e.g., less than about 5 vol. %, where porosity is measured as apercentage of pore volume divided by total layer volume. Further, insome examples, a substantially dense microstructure is a microstructurein which pores are closed (not interconnected), such that substantiallyno gas path from the outer surface of first layer 38 to the innersurface of first layer 38 exists for gases or vapors to move throughfirst layer 38.

For example, computing device 22 may control the feed rate to berelatively low, such that substantially all of the first coatingmaterial that coating material source 26 provides to plasma spray device20 is vaporized. When first layer 38 is deposited from substantiallyfully vaporized first coating material, the resulting microstructure offirst layer 38 may be substantially dense.

The technique of FIG. 5 also includes controlling, by computing device22, plasma spray device 20 to deposit first layer 38 including a rareearth disilicate on substrate 16 (76). As described above, first layer38 may include a rare earth disilicate formed by reaction of the silicaand the at least one rare earth oxide. During the PS PVD technique, thefirst coating material may be introduced into plasma 28, e.g.,internally or externally to plasma spray device 20. In PS PVD, vacuumchamber 12 is at a pressure lower than that used in low pressure plasmaspray. For example, as described above, computing device 22 may controlvacuum pump 24 to evacuate vacuum chamber 12 to a high vacuum with apressure of less than about 10 torr (about 1.33 kPa). In contrast, inlow pressure plasma spray, the pressure in a vacuum chamber is betweenabout 50 torr (about 6.67 kPa) and about 200 torr (about 26.66 kPa).Because of the lower operating pressure, the plasma may be larger inboth length and diameter, and may have a relatively uniform distributionof temperature and particle velocity.

The temperature of plasma 28 may, in some examples, be above about 6000K, which may result in vaporization of substantially all (e.g., all ornearly all) of the coating material, depending upon the rate ofintroduction of the coating material to the plasma 28. Plasma 28 maycarry the coating material toward substrate 16, where the coatingmaterial deposits in a layer on substrate 16. Because the coatingmaterial is carried by plasma 28 toward substrate 16, PS PVD may providesome non line-of-sight capability, depositing coating material on atleast partially obstructed surfaces (surfaces that are not in directline of sight with plasma spray device 20). This may facilitate formingfirst layer 38 on substrates with more complex geometry (e.g.,non-planar geometry).

The technique of FIG. 5 additionally may include, controlling, bycomputing device 22, coating material source 26 to provide a secondcoating material to plasma spray device 20 at a second feed rate (78).As described above, the second coating material may include silica andat least one rare earth oxide, or may include a thermal barrier coatingcomposition comprising a base oxide comprising zirconia or hafnia, aprimary dopant comprising ytterbia, a first co-dopant comprisingsamaria, and a second co-dopant comprising at least one of lutetia,scandia, ceria, gadolinia, neodymia, or europia.

When the first coating material includes silica and the at least onerare earth oxide, the amount of the at least one rare earth oxide andthe amount of silica may be selected so that second layer 40 depositedfrom the second coating material includes a predetermined ratio of theat least one rare earth oxide and silica. In some examples, due to thedifferences in vapor pressure between rare earth oxides and silica, theratio of the at least one rare earth oxide and silica in the secondcoating material provided by coating material source 26 may includeadditional silica compared to the composition of second layer 40, asdescribed above.

The excess silica in the second coating source may facilitate formationof second layer 40 including a rare earth monosilicate. By includingexcess silica in a predetermined amount, second layer 40 may be formedwith a desired amount of silica. For example, the excess amount ofsilica may be selected such that the ratio of silica to rare earth oxidedeposited in second layer 40 is substantially the same as astoichiometric ratio of the rare earth monosilicate. In other examples,the amount of silica in the coating material may be selected to resultin a predetermined amount of excess silica or excess rare earth oxide insecond layer 38 compared to a stoichiometric ratio of rare earth oxideto silica in the rare earth monosilicate. This may result in a selectedamount of free silica or free rare earth oxide in second layer 40.

The second feed rate may be selected so PS PVD of the second coatingmaterial results in second layer 40 including a columnar microstructure.For example, computing device 22 may control the feed rate to berelatively higher, such that not all of the second coating material thatcoating material source 26 provides to plasma spray device 20 isvaporized. When second layer 40 is deposited from not fully vaporizedsecond coating material, the resulting microstructure of second layer 40may be columnar. The technique of FIG. 5 also includes controlling, bycomputing device 22, plasma spray device 20 to deposit second layer 40including a rare earth disilicate on first layer 38 (80).

In some examples, the second layer may include the rare earthmonosilicate, and the technique of FIG. 5 may further, optionally,include controlling, by computing device 22, the coating material source26 to provide a third coating material to plasma spray device 20 at athird feed rate. The third coating material may have a compositionselected so that a third layer formed on the second layer from the thirdcoating material includes the thermal barrier coating compositioncomprising the base oxide comprising zirconia or hafnia; the primarydopant comprising ytterbia; the first co-dopant comprising samaria; andthe second co-dopant comprising at least one of lutetia, scandia, ceria,gadolinia, neodymia, or europia. In such examples, the technique of FIG.5 may further include controlling, by computing device 22, plasma spraydevice 20 to deposit the third layer on the second layer using PS PVD.

In some examples, computing device 22 may control coating materialsource 26 and plasma spray device 20 to deposit alternating layers ofthe rare earth monosilicate and the thermal barrier composition, eachlayer having a columnar microstructure.

In some examples, during the PS-PVD technique, computing device 22 maycontrol plasma spray device 20, stage 14, or both to move plasma spraydevice 20, substrate 16, or both relative to each other. For example,computing device 22 may be configured to control plasma spray device 20to scan the plasma plume relative to substrate 16.

The techniques described in this disclosure may be implemented, at leastin part, in hardware, software, firmware, or any combination thereof.For example, various aspects of the described techniques may beimplemented within one or more processors, including one or moremicroprocessors, digital signal processors (DSPs), application specificintegrated circuits (ASICs), field programmable gate arrays (FPGAs), orany other equivalent integrated or discrete logic circuitry, as well asany combinations of such components. The term “processor” or “processingcircuitry” may generally refer to any of the foregoing logic circuitry,alone or in combination with other logic circuitry, or any otherequivalent circuitry. A control unit including hardware may also performone or more of the techniques of this disclosure.

Such hardware, software, and firmware may be implemented within the samedevice or within separate devices to support the various techniquesdescribed in this disclosure. In addition, any of the described units,modules or components may be implemented together or separately asdiscrete but interoperable logic devices. Depiction of differentfeatures as modules or units is intended to highlight differentfunctional aspects and does not necessarily imply that such modules orunits must be realized by separate hardware, firmware, or softwarecomponents. Rather, functionality associated with one or more modules orunits may be performed by separate hardware, firmware, or softwarecomponents, or integrated within common or separate hardware, firmware,or software components.

The techniques described in this disclosure may also be embodied orencoded in a computer system-readable medium, such as a computersystem-readable storage medium, containing instructions. Instructionsembedded or encoded in a computer system-readable medium, including acomputer system-readable storage medium, may cause one or moreprogrammable processors, or other processors, to implement one or moreof the techniques described herein, such as when instructions includedor encoded in the computer system-readable medium are executed by theone or more processors. Computer system readable storage media mayinclude random access memory (RAM), read only memory (ROM), programmableread only memory (PROM), erasable programmable read only memory (EPROM),electronically erasable programmable read only memory (EEPROM), flashmemory, a hard disk, a compact disc ROM (CD-ROM), a floppy disk, acassette, magnetic media, optical media, or other computer systemreadable media. In some examples, an article of manufacture may compriseone or more computer system-readable storage media.

EXAMPLES Example 1

FIG. 6 is a scatter diagram illustrating an example relationship betweenexcess silica in a coating material and an amount of silicon in aresulting coating. The units on the x-axis of FIG. 6 are weight percentexcess silica (SiO₂) in the coating material. Excess silica is definedwith reference to a stoichiometric amount of silica in the coatingmaterial. In this example, the coating material includes a mixture ofsilica, ytterbium disilicate (Yb₂Si₂O₇), and alumina. For the sampleswith 5 wt. % excess silica, the mixture included 1 wt. % alumina and abalance ytterbium disilicate. For the samples with 5 wt. % excesssilica, the mixture included 3 wt. % alumina and a balance ytterbiumdisilicate. The units on the y-axis of FIG. 6 are weight percentelemental silicon (Si) in the resulting coating, as measured by amicroprobe. A stoichiometric ytterbium disilicate would include about10.9 wt. % silicon. As shown in FIG. 6, increasing excess silica in thecoating material generally increased the amount of silicon in theresulting coating.

Example 2

FIG. 7 is a cross-sectional picture of an example coating depositedusing PS-PVD as described in this disclosure. The coating illustrated inFIG. 7 was deposited from a coating material including 5 wt. % excesssilica, 1 wt. % alumina and a balance ytterbium disilicate. The PS-PVDparameters included using a He carrier gas with oxygen introduced intothe PS-PVD chamber. The power was about 119.9 kW, and the coating timewas about 12 minutes and 30 seconds. The coating was applied directly tothe substrate with a line-of-sight relationship. As shown in FIG. 7,with these conditions, the coating included a columnar microstructure.

Example 3

FIG. 8 is a cross-sectional picture of an example coating depositedusing PS-PVD as described in this disclosure. The coating illustrated inFIG. 8 was deposited from a coating material including 5 wt. % excesssilica, 3 wt. % alumina and a balance ytterbium disilicate. The PS-PVDparameters included using a carrier gas with oxygen introduced into thePS-PVD chamber. The power was about 116.6 kW, and the coating time wasabout 12 minutes. The coating was applied directly to the substrate witha line-of-sight relationship. As shown in FIG. 8, with these conditions,the coating included a porous microstructure, including closed poresthat do not extend through the thickness of the coating.

Example 4

FIG. 9 is a cross-sectional picture of an example coating depositedusing PS-PVD as described in this disclosure. The coating illustrated inFIG. 9 was deposited from a coating material including 5 wt. % excesssilica, 3 wt. % alumina and a balance ytterbium disilicate. The PS-PVDparameters included using a He carrier gas with no oxygen introducedinto the PS-PVD chamber. The power was about 117.0 kW, and the coatingtime was about 17 minutes and 30 seconds. The coating was applieddirectly to the substrate with a line-of-sight relationship. As shown inFIG. 9, with these conditions, the coating included a substantiallydense microstructure.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. An article comprising: a substrate defining atleast one at least partially obstructed surface, wherein the substratecomprises at least one of a ceramic or a ceramic matrix composite; and amultilayer, multi-microstructure environmental barrier coating on the atleast partially obstructed substrate, wherein the multilayer,multi-microstructure environmental barrier coating comprises: a firstlayer comprising a rare earth disilicate, between about 0.1 wt. % andabout 3 wt. % alumina, and a microstructure having less than about 10vol. % porosity; and a second layer on the first layer, wherein thesecond layer comprises a columnar microstructure, a rare earthmonosilicate, and between about 0.1 wt. % and about 3 wt. % alumina; athird layer on the second layer, wherein the third layer comprises athermal barrier coating composition comprising a base oxide comprisingzirconia or hafnia; a primary dopant comprising ytterbia; a firstco-dopant comprising samaria; and a second co-dopant comprising at leastone of lutetia, scandia, ceria, gadolinia, neodymia, or europia.
 2. Thearticle of claim 1, wherein the first layer consists essentially of therare earth disilicate and alumina.
 3. The article of claim 1, wherein atleast one of the first layer or the second layer further comprises atleast one alkali oxide or at least one alkaline earth oxide.
 4. Thearticle of claim 1, wherein the first layer comprises between about 0.5wt. % alumina and about 3 wt. % alumina.
 5. The article of claim 1,wherein the second layer comprises between about 0.5 wt. % alumina andabout 3 wt. % alumina.